Along with the miniaturization of semiconductor devices, the trend is moving into shorter wavelength of the exposure light source and higher numerical aperture (high NA) of the projection lens. At present, an exposure machine with NA of 0.84 has been developed, where an ArF excimer laser having a wavelength of 193 nm is used as the light source. As commonly well known, these can be expressed by the following formulae:(Resolving power)=k1·(λ/NA)(Focal depth)=±k2·λNA2wherein λ is the wavelength of the exposure light source, NA is the numerical aperture of the projection lens, and k1, and k2 are constants related to the process.
In order to realize still shorter wavelength and higher resolving power, studies are being made on an exposure machine where an F2 excimer laser having a wavelength of 157 nm is used as the light source. However, the lens material used for the exposure apparatus so as to realize shorter wavelength and the material used for the resist are very limited and therefore, it is extremely difficult to stabilize the production cost or quality of the apparatus and materials. This may lead to a failure in procuring the exposure apparatus and the resist each assured of sufficiently high performance and stability within a required time period.
Conventionally, a so-called immersion method of filling a high refractive-index liquid (hereinafter sometimes referred to as an “immersion liquid”) between the projection lens and the sample has been known as a technique of increasing the resolving power in an optical microscope.
As for the “effect of immersion”, assuming that the wavelength of exposure light in air is λ0, the refractive index of the immersion liquid to air is n, the convergence half-angle of beam is θ and NA0=sinθ , the above-described resolving power and focal depth when immersed can be expressed by the following formulae:(Resolving power)=k1·(λ0/n)/NA0 (Focal depth)=±k2·(λ0/n)/NA02 
That is, the effect of immersion is equal to use of an exposure wavelength of 1/n. In other words, in the case of a projection optical system with the same NA, the focal depth can be made n times larger by the immersion. This is effective for all pattern profiles and can be combined with super-resolution techniques such as phase-shift method and modified illumination method which are being studied at present.
Examples of the apparatus where this effect is applied to the transfer of a fine image pattern of a semiconductor device are described in JP-A-57-153433 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A-7-220990.
Recent progress of the immersion exposure technique is reported, for example, in SPIE Proc., 4688, 11 (2002), J. Vac. Sci. Tecnol. B, 17 (1999), SPIE Proc., 3999, 2 (2000), JP-A-10-303114 and International Publication No. WO 2004-077158. In the case of using an ArF excimer laser as the light source, in view of safety on handling as well as transmittance and refractive index at 193 nm, pure water (refractive index at 193 nm: 1.44) is considered to be a most promising immersion liquid. In the case of using an F2 excimer laser as the light source, a fluorine-containing solution is being studied in the light of balance between transmittance and refractive index at 157 nm, but those satisfied in view of environmental safety or refractive index have been not yet found out. Considering the degree of immersion effect and the maturity of resist, the immersion exposure technique is expected to be most soon mounted on an ArF exposure machine.
Since the discovery of a resist for a KrF excimer laser (248 nm), an image forming method called chemical amplification is used as the image forming method for a resist so as to compensate the reduction in the sensitivity due to light absorption. The image forming method, for example, using positive chemical amplification is an image forming method where an acid generator in the exposed area decomposes upon exposure to generate an acid, the acid generated is used as a reaction catalyst in the baking after exposure (PEB: post exposure bake) to convert the alkali-insoluble group into an alkali-soluble group, and the exposed area is removed by an alkali developer.
As for the acid generator which is the main constituent component of the chemical amplification-type resist composition, various compounds have been found. For example, JP-A-2004-002252 discloses an onium salt capable of generating a sulfonic acid upon irradiation with radiation.
Also, for example, JP-A-2003-261529, U.S. Patent Application Publication 2003/0148211A, U.S. Pat. No. 5,554,664, JP-A-2002-341539 and JP-A-2002-268223 disclose a photosensitive composition containing a sulfonium or iodonium salt having a bis-sulfonylimide or tris-sulfonylmethide anion.
However, many points still remain unsatisfied, and improvement is demanded with respect to the line edge roughness (LER).
The line edge roughness as used herein means a profile such that the resist edge at the interface between the line pattern and the substrate irregularly fluctuates in the direction perpendicular to the line direction due to the resist characteristics. When this pattern is viewed from right above, the edge gives an uneven appearance (on the order of ±several nm to several tens of nm). This unevenness is transferred by the etching step and when the unevenness is large, failure in electric characteristics is caused and the yield decreases.
On the other hand, it is pointed out that when immersion exposure is applied to the chemical amplification resist, the resist layer comes into contact with the immersion liquid at the exposure, as a result, the resist layer deteriorates or a component adversely affecting the immersion liquid bleeds out from the resist layer. International Publication No. WO 2004-068242 describes a case where when the resist for ArF exposure is dipped in water before and after exposure, the resist performance is changed, and this is indicated as a problem in the immersion exposure.
For example, in the immersion exposure, the line width is known to fluctuate due to time delay between exposure and PEB, and improvement is demanded in this respect.